1. Field of the Invention
This invention relates to a method of compressing and reconstructing image signals. This invention particularly relates to a method of compressing and reconstructing image signals by vector quantization.
2. Description of the Prior Art
Image signals representing half tone images, such as television signals, are composed of enormous amounts of information, and a broad-band transmission line is required for transmission of the image signals. Such image signals involve much redundancy, and various attempts have been made to compress the image signals by restricting the redundancy. Also, in recent years, recording of half tone images on optical disks, magnetic disks, or the like has been generally put into practice. In this case, image signal compression is conducted generally for the purpose of efficiently recording image signals on a recording medium.
One of the methods of image signal compression that has heretofore been known is a method wherein vector quantization is utilized. The known method comprises the steps of (i) dividing two-dimensional image signals into blocks each comprising a fixed number of samples, (ii) selecting a vector that corresponds with the minimum distortion to the set of the image signals in each of the blocks from a code book comprising a plurality of vectors different from one another and prepared in advance by defining the fixed number of vector elements, and (iii) encoding the information representing the selected vector to correspond to the block.
Since the image signals in the block as mentioned above have high correlation therebetween, the image signals in each block can be represented very accurately by one of a comparatively small number of vectors prepared in advance. Therefore, transmission or recording of the image signals can be carried out by transmitting or recording a code representing the vector, instead of the image signals themselves, and signal compression can thus be achieved. By way of example, the amount of the image signals at 64 picture elements in a half tone image of a density scale composed of 256 levels (=8 bits) is 8.times.64=512 bits. In the case where the 64 picture elements are grouped as a single block, the respective image signals in the block are expressed by a vector composed of 64 vector elements, and a code book including 256 such vectors is prepared, the amount of the signals per block becomes equal to the amount of the signals for discrimination between the vectors, i.e. 8 bits. Consequently, in this case, the amount of the signals can be compressed to 8/(8.times.64)=1/64.
After the image signals are compressed in the manner as mentioned above and recorded or transmitted in the compressed form, the vector elements of each of the vectors which the vector discriminating information represents are taken as reconstructed signals of each of the blocks, and the original image is reproduced by use of the reconstructed signals.
On the other hand, in the case where the image signals are compressed and reconstructed in the manner as mentioned above, the vector elements of each vector are often defined at levels exceeding the resolution of an image reproducing apparatus. Specifically, by way of example, the density resolution of the image reproducing apparatus may be 256 levels (for simplicity of explanation, the levels are assumed to range from 1 to 256), and the vector elements may be defined by numerals of 25,600 levels (for simplicity of explanation, the levels are assumed to range from 1.00 to 256.00). In such a case, the lower order part of each of the vector elements has heretofore been raised or discarded. Specifically, in the aforesaid example, when the vector elements attains a value of 25.65, the density level has heretofore been taken always as 26 or 25. However, when such raising or discarding is carried out, the image quality of the reproduced image is deteriorated.